The invention pertains generally to the electrical system of an automotive vehicle and, more particularly, to control circuitry for controlling voltages within such systems in which the vehicle battery is switchably coupled to the electric alternator.
It has been found useful to employ the standard alternator of an automotive electrical system for purposes other than supplying current to the battery and standard automotive load. One improtant use is to energize heating elements embedded within the windshield for deicing purposes. In such application, the alternator output is switchably connected to the battery and heating elements so as to supply a relatively high power to the heating elements during heating, while continuing to perform its prime function of supplying current to the battery and standard load. In this type of operation, during non-heating periods the battery will receive charge current from the alternator, and the voltage across it and the shunt connected load circuit is regulated to the battery fully charged or float voltage. During heating periods the battery will supply some or all of the current requirements of the load, and the voltage across it and the load drops to the battery discharge level. Due to the internal resistance of the battery, the float and discharge voltages differ by several volts. Accordingly, as the alternator is switchably coupled between energizing the battery and energizing the heater elements, the voltage across the battery and standard load changes abruptly between the float and discharge levels. When the headlamps are on, this results in headlamp flare, i.e., a sudden brightening or dimming of the headlamps. In addition, when the blower is on, there is a noticeable change in blower sound.